1. Field of the Invention
This invention relates to a method and apparatus for sorting a particulate material into coarse and fine fractions.
2. Description of the Prior Art
In the past, separators have been developed of the type having a vertical, rotationally symmetrical, preferably cylindrically shaped separator wall, and a vane rotating therein about the cylindrical axis. In such a separator, the material is suspended in a vertically ascending conveying gas that is conveyed past the rotating vane. Centrifugal force generated by the rotating vane flings the coarser fraction of the material in a radially outwardly direction toward the wall, to be passed down towards the bottom and out of the separator. The finer fraction of the material is passed on upwards by the conveying gas to be subsequently separated from the gas.
As a result of this separating method and apparatus, the fine fraction practically comprises all of the grains in the suspension that are below a predetermined smaller first grain size, and the coarse fraction practically comprises all of the grains that are above a predetermined second larger grain size. An intermediate fraction comprising grain sizes between the said first and second grain sizes is present both in the fine and the coarse fraction at an increasing percentage of larger and larger grains in the coarse fraction and a correspondingly declining percentage in the fine fraction. This distribution of the intermediate fraction in the fine and the coarse fraction respectively, is due to the fact that the centrifugal forces acting upon the grains as a consequence of the rotary vane are different dependant upon the position of the grains in the suspension in relation to the axis of rotation. The tendency of the grains in the intermediate fraction to being sorted to the coarse fraction will thus increase the larger the distance from the axis of rotation when they reach the vane.
In principle the size of the difference between the above first and second grain sizes expresses the separator's sorting capability or separation sharpness. The smaller this difference, the better the separation sharpness, and the better the separation of the suspension into two fractions.
I have invented a method and apparatus in which the separation sharpness of the above described technique is vastly improved. According to the invention this is achieved by providing a band of clean conveying gas extending annularly around the suspension gas flow within the separator.